The book has some value, propagates some interesting ideas and is written with an engaging sense of style. I am always happy to see books written that espouse the truth of the value of darkness and the problems that duality (good/light and dark/bad) presents, and how it's made our culture intensely one-sided.
However, I felt the book was written with a more "new age" bent and did not adequately prepare the reader for the very real dangers of darkness meditation and working with what comes out of the unconscious as a result of it. There is a reason that both monsters and treasures, Tartarus and Elysium exist in the darkness of Hades together- darkness brings out a variety of things we term "good" and "evil", and not all of them want to give you a hug and assure you that you're a special and unique snowflake. Also, I wasn't thrilled with some of their mythological references, which showed they didn't have a very thorough understanding of said mythologies beyond what is popularly represented: such as regarding humans as merely the "playthings" of the gods (anyone with an understanding of the Orphic cults can see through that one). Also, the exercises are incredibly cookie-cutter, and remind me of a Terry Lynn Taylor book about angels that I read in college.
So, for the serious practitioner or for someone looking for a book with real bite, I would suggest you seek elsewhere.